This invention relates to the field of frequency modulators and in particular to those modulators having a voltage controlled oscillator (VCO) for modulating a resonant frequency.
Frequency modulated (FM) signals are widely used in many communication devices, such as two-way portable and mobile radios, for communicating messages and other types of information. The FM signals are generated by varying the frequency of a carrier signal according to the variations of a modulation signal. The modulation signal may be an analog or a digital signal which is generated by a message transducer, such as a microphone, or a binary device, such as a microprocessor.
Generally, the modulation signal is applied to a frequency modulator which produces the FM signal. The FM signal may be processed in accordance with the requirements of a particular application to recover the modulation signal. For example, in a radio frequency (RF) transmitter application, the FM signal is amplified and transmitted through an antenna to other receiving devices which include means for recovering the modulation signal.
Many of the conventionally used frequency modulator circuits comprise voltage controlled oscillators. A VCO, which includes a feed back loop, provides a resonant frequency in response to a control voltage applied at a control port. Typically, a VCO circuit includes a varactor coupled to the control port and a resonator stage. The resonator stage is coupled to an amplifier stage, the output of which is fed back to the resonator stage so as to cause oscillation at the resonant frequency. As such, the resonant frequency of the VCO may be varied according to the variations of the control signal.
The VCO, when used as an FM modulator, is the modulation signal through a modulation port which is capacitively coupled to the resonator stage through a modulation varactor. Conventionally, the modulation signal may be coupled to the VCO in a number of different ways. In some applications, the modulation signal is applied directly to the control voltage port, thus, using the control voltage varactor as a modulation varactor as well. However, this approach makes the FM signal susceptible to variations on the control line and adversely affects the loop response of the VCO. These adversely affected parameters are manifested by an increase in side band noise or severe hum and noise degradations.
In another approach, the modulating signal is applied through a modulating port which includes an independent modulation varactor. The modulation varactor is coupled to the resonator stage by means of a capacitive coupling network. Thus, variations of the modulation signal varies the capacitance across the resonator, thereby providing a modulated signal which is amplified by the amplifier stage to produce the FM signal. However, the capacitive network of this approach causes the resonant frequency to suddenly shift to an undesired center frequency.
In yet another approach, the modulation signal is applied to the incoming supply line of the VCO through an interfacing circuit. However, this approach makes the FM signal susceptible to supply line variations.
Thus, conventional approaches for coupling the modulation signal to the VCO may adversely affect the resonator's quality factor (Q), and/or the loop filter response of the VCO, resonant frequency, and/or the side-band noise.